The present invention relates to a base station apparatus equipped with an array antenna that has a plurality of antennas used in a digital radio communication system, and a radio communication method.
In a digital radio communication system, adaptive radio transmission technology is applied and an array antenna that has a plurality of antenna elements is used. A linear array antenna is used as one kind of array antenna. This linear array antenna has a configuration in which a plurality of antenna elements are placed in a straight line at intervals of half the wavelength of the carrier frequency.
With a linear array antenna, complex amplitude multiplication is performed for received signals that are input via the respective antennas, to give an arbitrary directivity. This technology has been disclosed in Unexamined Japanese Patent Publication No. 9-284200. Here, as an example of a linear array antenna, a base station apparatus equipped with a linear array antenna comprising four antennas arranged in a straight line will be described.
FIG. 1 is a block diagram showing the configuration of a base station apparatus equipped with a conventional linear antenna. With this base station apparatus, four antennas 1 to 4 receive radio signals, the prescribed radio reception processing (down-conversion, A/D conversion, etc.) is performed for the respective radio signals by RF sections 5 to 8 provided for each antenna, and a signal in the stipulated frequency band or intermediate frequency band is obtained. Then, in this base station apparatus, this signal is subjected to demodulation processing and receive data is obtained.
With regard to transmit data, on the other hand, after digital modulation by a modulation section 10 for the respective users, these post-modulation signals are linearly added, the prescribed radio transmission processing (D/A conversion, up-conversion) is performed by RF sections 5 to 8, and the signals are transmitted to antennas 1 to 4.
When a linear antenna is used in a base station apparatus, when communication is performed the signal power in a specific direction is adjusted, giving arbitrary directivity. In this case, a weight vector product section 9 finds the complex amplitude product for the received signals from each antenna, and gives an arbitrary directivity using the result. For example, if communication is performed with a mobile station 12 in direction xcex8 from a base station apparatus 11, as shown in FIG. 2, the product is found of the complex amplitude vector shown in expression (1) below for the received signals in the order of the antennas of the base station apparatus 11.
W=[1,exp(xe2x88x92jxcfx80 sin xcex8),exp(xe2x88x92j2xcfx80 sin xcex8),exp(xe2x88x92j3xcfx80 sin xcex8)]Txe2x80x83xe2x80x83expression (1)
By performing the above kind of vector multiplication for the received signals, the signal transmitted from the base station apparatus 11 has maximum power in the xcex8 direction, and it is possible to form a power distribution (beam) that becomes weaker with increasing distance from xcex8. With four linear array antennas, it is possible to drop the signal power by half or more when xcex8xc2x122.5 is exceeded.
The formation of this power distribution is called beam forming in the xcex8 direction. By means of this beam forming, it is possible to increase the signal power for the desired signal, so that in communication with a certain user there is little susceptibility to influence by received signals from other user directions, and it is also possible to reduce the power of transmitted signals to users in other directions that constitute unwanted waves. It is possible, for example, to perform simultaneous processing as shown in FIG. 2, with beam 14 in the xcex8 direction used for communication with mobile station 12 in the xcex8 direction, and beam 15 in the xcfx86 direction used for communication with mobile station 13 in the xcfx86 direction.
By this means, it is possible to expect improved quality of communication with users, and increased communication capacity of the system as a whole.
However, the following kinds of problem arise in transmit operations using a linear array in the above kind of base station apparatus. In vector multiplication in the process of beam forming in a base station apparatus, the signal received from a specific antenna is normally used as a reference, and this signal is multiplied by a complex amplitude in which the real part is 1 and the imaginary part is 0. In expression (1) above, the complex amplitude multiplied for the first antenna has real part 1 and imaginary part 0.
Therefore, in this vector multiplication, whereas the real part is 1 and the imaginary part 0 for a beam in any direction for the first antenna, for the other antennas a component emerges that is cancelled in multiplexing of beams from phase rotation. As a result, when this kind of vector multiplication is performed, the transmission power increases only for the first antenna, and with 50 users, for example, has the power of 50 signals, and the dynamic range of the transmission amplifier must be increased.
It is an objective of the present invention to provide a base station apparatus and radio communication method that enable directivity forming to be performed in transmission using an array antenna that has a plurality of antenna elements, and that moreover enable the transmission amplifier load to be alleviated.
A main subject of the present invention is to apply the same phase rotation to each element of a weight vector in a specific direction, to disperse the antenna elements giving the maximum or minimum amplitude among individual users, and decrease the amplitude bias toward a specific antenna element, and thereby to alleviate the load on the transmission amplifier.